Radial-flow molecular pump



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s H. RV: ra I my... r u 2 w 1E 0% 1, 0 X J r P T United States Patent Ofi" 3,322,334 Patented May 30, 1967 3,322,334 RADIAL-FLOW MOLECULAR PUMP Eldon L. Knuth, Pacific Palisades, and Sam F. Iacohellis, Woodland Hills, Calif, assignors to The Regents of the University of California, a corporation of California Filed July 14, 1964, Ser. No. 382,463 7 Claims. (*Cl. 230131) The present invention relates generally to pumps for pumping gaseous fluids; and is more particularly concerned with radial-flow, high-vacuum or molecular pumps.

There are various commercial situations as well as experimental works being carried on in connection with space environment simulators which require pumping large volumes of gases at low pressures. For example, in order to maintain an experimental space system at a very low pressure, say, 10- millimeters of mercury or less, simulating pressure in outer space, it may be necessary that extremely large volumes of gases be pumped. In general, the problems encountered and the methods employed in vacuum technology are substantially different from those found in other branches of engineering.

Many different types of pumps are normally available for producing a vacuum. Because of the limitations of each different type, it has often been found expedient to combine two or more kinds in series. For exanrple, reciprocating positive displacement pumps with contracting and expanding chambers have been developed which are able to produce pressure ratios between outlet and inlet of the general order of 100ml in a single stage; but their uses are generally limited by low displacement capacity for any given size and by inability to pump with inlet pressure below about microns or 10 millimeters of mercury. Pumps of this type are inherently unbalanced and thus have limitations as to speed.

In order to'reduce the problems of balance and low speed, rotary types of pumps have been developed which, being balanced, can be operated at higher speeds than reciprocating pumps and, accordingly, higher flow capacities are thus obtained. However, rotary pumps also are not well adapted to operating with their inlet at a high vacuum, since the pumps are not efficient below an inlet pressure of the general order of 10 microns or 10* millimeters of mercury, i.e., at lower inlet pressures, rotary pumps also begin to lose their pumping ability.

Very high flow rates have been achieved with oil diffusion pumps which operate etficiently at pressures below 10 microns, but operate over only a rather narrow range of inlet pressures. Diffusion pumps lose their effectiveness at inlet pressures higher than approximately 10 microns pressure or lower than approximately 0.01 micron.

There are still other types of pumps employed to produce a high vacuum, but they each have their own limitations which, in general, limit their volumetric capacity to values below those required by space simulators and contemplated for the present invention.

Bladed axialand radial-flow pumps have not been used extensively in vacuum work, although they are very widely used to pump large volumes at atmospheric or slightly higher pressures. Pumps of this type have not been considered as applicable to vacuum technology because of their relatively low pressure ratio per stage, the pressure ratio usually not exceeding a maximum of 2.0 per stage. This limitation of such pumps indicates very strongly that too many stages would be required to attain the extremely high overall pressure ratio required to produce an acceptable high vacuum. A particular advantage of radial-flow pumps is their ability to handle high flow rates within a single unit; and this capability makes them attractive for applications such as are here contemplated. Hence it was decided to investigate in detail this type of pump for vacuum pumping in spite of the fact that the geometry of some such pumps makes them obviously unsuited.

Thus it is a general object of the invention to provide a pump for vacuum work having: a high volumetric capacity and operating eflectively at .inlet pressures below about 10- millimeters of mercury.

Differently stated, it is an object of the invention to produce a novel design of a pump for gaseous fluids having a high volumetric flow rate capable of being com bined in series with other known types of pumps to produce high vacuum.

More specifically, it is an object of the invention to produce a novel design of a radial-flow pump having high volumetric capacity and adapted to work effectively at low inlet pressures in the range of 10* millimeters of mercury or substantially less.

These objects are achieved according to the present invention by providing a radial-flow vacuum pump comprising a rotor having a pair of spaced end walls and a plurality of blades extending axially between the end walls and arranged in an annular series around the rotor. A plurality of stator blades is also arranged concentrically of and downstream from the rotor. The blades on the rotor are relatively thin, substantially flat blades disposed at an angle in the range of about 5 degrees to about 25 degrees, and preferably not more than about 20 degrees with a tangent at the center of the blades, While the stator blades are similar in shape and oppositely, though not necessarily equally, inclined to those on the rotor. The stator blades preferably make an angle with a median tangent about equal to the angle of the rotor blades, or sometimes a little less. Motor means is provided driving the rotor at a tangential velocity in excess of about 400 feet per second. Blade speeds in the range of 400 to 1,000 feet per second are highly effective for pumping without imposing undue stresses on the rotor, which is constructed of good strength aluminum stock. Constructing the rotor of advanced materials will allow even higher blade speeds and more effective pumping.

How the above objects and advantages of the present invention, as well as others not specifically referred to herein, are attained will be more readilyunderstood by reference to the following description and to the annexed drawings, in which:

FIG. 1 is a diagrammatic view of laboratory apparatus including a radial-flow vacuum pump, embodying the present invention, in series with a diffusion pump;

FIG. 2 is a diagram illustrating an alternative arrangement of the pump constituting the present invention permitting a different relationship between the pump and the chamber to be evacuated;

FIG. 3 is an enlarged combined end elevation and partial section of the pump;

FIG. 4 is a fragmentary longitudinal section on line 44 of FIG. 3;

FIG. 5 is a perspective of the assembled rotor and stator with the motor means for driving the rotor, at reduced scale and removed from the chamber;

FIG. 6 is a diagram illustrating the blade geometry and nomenclature for rotor and stator blades;

FIG. 7 is a graph of calculated performance characteristics of a single-stage pump; and

FIGS. 8 to 12 are diagram illustrating various possible configurations of the rotor and stator for different sup porting arrangements and direction of gas flow.

Referring now to the drawings, and particularly to FIG. 1, there is illustrated a preferred embodiment of the pump. The wall means indicated generally at 10 encloses the pump and is preferably cylindrical in shape in order to withstand more efficiently the pressure differential creas ated by the low internal pressure. The wall means defines two chambers, one chamber 11 containing the pump, while the other chamber 12 is a test chamber. Wall means serves to isolate the pump from the atmosphere and delines a low pressure chamber or environment within which the pump operates. In the broad aspect of the invention, the shape of the chamber is immaterial as the wall means 10 has no fixed relation to the pump beyond enclosing a space having a suitable pressure; and test chamber 12 is merely a part of exemplary apparatus with which the pump may be used, and may be omitted. Further, as will become apparent, the pump may pump into or out of the space around the pump but inside chamber 11.

As may be seen in FIG. 1, chamber 11 has an outlet 14 which is connected to an oil diffusion pump 15 which may be of any suitable design, such pumps being well known in the art. The outlet of the diffusion pump is, in turn, connected to the inlet of a rotary pump 16, which likewise may be of any suitable type or design.

The pump indicated generally at in FIG. 1 is shown in perspective in FIG. 5 removed from chamber 11 and in greater detail in FIGS. 3 and 4. Pump 20 has at each end a stationary spider comprising a plurality of radially extending arms or spokes 21 connected at their inner ends to a hollow, stationary hub 22. At their outer ends, arms 21 of each spider are connected to and support annular plate 24. At one end, the right-hand end in FIG. 4, spokes 21 are connected directly to plate 24, while at the other end, the left hand end in FIG. 4, the spokes are each connected to the plate 24 through an angle bracket 25 which spaces spokes 21 from the plate, for reasons which will become apparent.

Each of the spider hubs 22 is provided with a ball hearing 27, upon the outer race of which is mounted hollow hub 28 of the rotor indicated generally at 29. At one end, such as the left-hand end in FIG. 4, hub 28 is provided with an axially projecting portion 30 which serves as a drive pulley around which passes a V-belt 31 driven by pulley 32 on the output shaft of motor 34. Motor 34 is rigidly mounted between two successive spokes of spider 21 by mounting bracket 35, locating drive pulley 32 in the space between spider arms 21 and annular plate 24. Although only a single motor is shown here, it has been found desirable under some circumstances, in order to obtain additional power, to provide a second motor which can be similarly supported between two other spokes of the end spider and still drive the rotor from the same hub with a second V-belt.

Each of rotor hubs 28 has attached to it a radially extending disc 38 inwardly of and closely spaced from the stationary plate 24. Thus the two discs 38 provide a pair of spaced end walls at opposite ends of the rotor upon which the rotor blades are supported. The rotor has a plurality of axially extending blades 40 and 41, that is, these blades have their longitudinal axes extending parallel to the axis of rotation of the rotor. Blades 40 are also arranged in an annular series or row around the periphery of the rotor. In a two-stage pump of the type shown in the exemplary embodiment, there is also a second annular series or row of blades 41 parallel to and radially inward from the outer series 40; but it will be understood that, in a single-stage pump, either blades 41 or 40 may be omitted and only one series of blades used.

Blades 40 and 41 are thin, substantially fiat, and relatively elongated, for reasons which will be discussed in greater detail later. Since the distance between end walls 38 is greater than can be spanned by blades 40 and 41 without intermediate supports required to prevent excessive deflection of the blades under stresses developed during rotation of the rotor, it is preferable to make the blades in short segments supported by spaced rings 43 and 44 for the outer and inner rows of rotor blades, respectively. The cross-sectional area of these rings and their axial spacing are selected to give the desired degree of rigidity to the rotor, and particularly to the blades. In

speaking of the blades herein, they are treated as extending from one end wall 38 to the other, since rings 43 and 44 are reinforcing hoops only and the segments are axially aligned to pump in the same way as a single blade of equal length, but with slightly less efficiency.

The rotor blades 40 and'41 are preferably flat, though they may be slightly curved transversely, if desired. Some gain in efliciency can be obtained under some conditions by the transverse curvature, a well as increased stiffness; but the fiat blades are easier to fabricate. The same is also true of the stator blades, to be described.

In abroad aspect, the rotor blades extend substantially between the two end walls provided by end plates 38; and this is true even though the rotor blade are not mounted on or supported by the end walls as in FIG. 9. The arrangement of the spider and discs for support as in FIGS. 3 and 4 is advantageous and preferred for radial outward flow; but the invention is not limited thereto. Hence the rotor blades are characterized as extending substantially between the end walls, even if they are physically discontinuous to provide for supports or are not connected to the end walls but spaced therefrom by running clearances. The same is also true generally of the stator blades since they are substantially co-extensive axially of the rotor blades and may in some situations be mounted on the end walls.

Downstream from each of the rotating rows of blades (excepting, in some cases, downstream from the final rotating row of blades) is a concentric row of stator blades. Since gas flow in the embodiment illustrated is designed to be in a direction from inside the rotor outwardly, stator blades 45 and 46 are located closely adjacent and radially outward from the rows of rotor blades 41 and 41, respec tively. However, this same rotor design may serve equally well for radially inward gas flow by reversing either the inclination of the rotor blades or the direction of their rotation. In that case, the stator blades, to be downstream from the associated rotor blades, would be radially inward from the blade rows 40 and 41, respectively. In the case in which gas flows from outside the rotor inwardly, pro-.

visions would be made for an adequate flow passage from the inside of pump 20 to the fore pump.

The stator blades, like the rotor blades, are thin, substantially flat, and elongated. Since they are not subjected to any dynamic loading, they may be thinner than the rotor blades. They are inclined like the rotor blades, but oppositely and not necessarily at the same angle of inclination. The rotor and stator blades are each arranged in annular series or rows, i.e., the blades of any one series or row have all corresponding points lying on a cylindrical surface.

In the outer row of stator blades 45, the blade segments are connected at their opposite ends to the stationary discs 24 which are supported by and upon the spiders at the end of the pump. In a single-stage pump, blades 45 can be continuous between the two discs 24. However, in a two-stage pump as here shown, the stator blades are in two segments with the inner ends of the two segments of blades 45 connected to annular plate 48 disposed in a radial plane to support this plate. The ring 48 is utilized to support the inner row of stator blades 46 at the adjacent ends of the blade segments, the plate 48 being the sole support in this construction for the inner row of stator blades. At their opposite outer ends, the stator blade segments are connected by rings 42 which space the blades and give rigidity to the blade assembly. The outer blades 45 are a single assembly supported at its ends while the inner blades are a second assembly supported centrally from the outer row.

The inner row of rotor blades 41 forms a single continuous assembly extending between the two supporting end walls 38 while the outer row of rotor blades 40 is divided into two physically distinct assemblies. Each of these assemblies of the blades 40 is supported from one of end walls 38, with the inner adjacent ends of the blade assemblies spaced apart in order to permit annular plate 48 to pass between them.

This means of supporting the blade assemblies permits a multiple-stage pump; and it will be realized that a third or even more stages can be added using this construction for the rotor and stator blades.

Arms 21 engage the wall of chamber 11 to support the pump within and spaced from the chamber wall. One end wall 11a of the chamber 11 is removable to permit the pump to slide in and out.

The pump described above is designed for radial outward gas flow, gas entering the rotor interior through an inlet at one end wall provided by the hollow hub 22. Because of the low absolute pressure differential between the interior of the pump and the surrounding space inside chamber 11, dynamic seals between the rotor and stator are unnecessary. Leakage paths exist between rings 49 and end walls 38 in the first stage; and between rings 24 and end Walls 38 and at support ring 48 in the second stage. In each location, the running clearance has been kept to about /8 inch. The theoretical leakage is of the general order of 4 percent by volume; but this is acceptable since the reduction in pressure ratio is less than 2 percent as a result. In actual practice it is felt that these values are probably upper limits on losses.

The apparatus, including the pump structure, illustrated in FIG. 1 was designed especially to admit into test chamber 12 a high velocity beam of gas, typically argon, projected along the axis of the rotor. This beam, from any suitable source, not shown, enters chamber 11 through duct 50. A portion of this beam passes through the hollow hubs of the pump stator in succession into the vacuum chamber 12. As the beam passes through the pump, a portion of the beam diffuses into the interior of the pump rotor and is pumped by the pump through the rotor and the stator into the space within chamber 11 but externally of the pump. From this space the gas is withdrawn by dilfusion pump 15 and rotary pump 16. The molecular beam is one example of a source of gas to be pumped, and the present invention is not limited thereto. Likewise, it is apparent that the test chamber 12 is not an essential part of the present invention and, accordingly, it is not described in detail.

It should be kept in mind in considering the design of this molecular pump that it is operating in a very rarified atmosphere. When the ratio of the mean free path of the molecules of the gas to a significant dimension of the structure in the fluid stream is equal to or greater than one, that is, when the length of paths between molecule-to-molecule encounters is comparable to or larger than a dimension of the flow region, the gas must be treated as consisting of discrete particles. Under these flow conditions, normally referred to as free-molecular flow, the usual concepts of continuum gas dynamics no longer apply. Under conditions of free-molecular flow, the pumping ability of the bladed rotor of the construction herein used arises from the fact that molecules incident on the moving blade from the low pressure side have a higher probability of proceeding through the rotor than do the molecules incident on the blade from the high pressure side.

Under the assumed, conditions of free-m olecular flow, each gas molecule behaves independently of each other gas molecule and the probability that a molecule striking a rotor blade will pass through the opening between the blades from the upstream to the downstream side is deter mined by the probability of its being reflected, after one or more collisions with the blades, at a suitable angle to pass through the rotor.

Let S be the fraction of all molecules incident on the blade row from the upstream side (1) that ultimately pass through to the downstream side (2); probably after several collisions with the blades. Similarly, S is then the fraction of the molecules incident on the blade from the downstream side passing through to the upstream side. Further, let N and N be the number of molecules, re

spectively, incident upon the blade .row per unit time per unit area from upstream and downstream, respectively. The net flux or flow of molecules through the row of blades is now expressed in terms of the ratio W, which is the ratio of the net throughflow, as molecules per unit time per unit area, to the incident flux N at the upstream side of the row. At steady state, conservation of molecules requires that which may be expressed as These probabilities can be calculated theoretically.

either by the use of integral equations or by using'a set of random numbers to establish an assumed molecule distribution at the opening between two blades of the rotor. Then the space coordinates and the directions of the velocity vector of an individual molecular incident upon the blades are generated, from which it is determined whether the molecule has a collision with the adjacent blade or hits an opening. If the molecule hits the blade, the coordinates of the collision point are calculated and a new r vector is calculated. Each molecule is thus followed as it enters a passage and undergoes successive reflections, finally emerging into either the upstream region or the downstream region. The fraction of all such molecules transmitted through the blades from upstream to downstream gives the probability of transmission S Based on theoretical performance of th free-molecular pump, the performance for various blade configurations was calculated. The analysis showed that a blade chordto-spacing ratio b/s, as shown in FIG. 6, within the range of approximately .5 to 2.0, but preferably about 1.0, with a blade angle a within therange of 5 degrees to 25 degrees but preferably equal to about 20 degrees, give a good blade design for a single-stage molecular pump. As shown in FIG. 6, ll plus degrees is the angle between the impact surface of a blade and a radius through the center of the blade. Thus, a must be regarded as the angle that the blade makes witha tangent through the center of the blade. The blade angle is a function of velocity and the value of 20 degrees has been selected as suited to a velocity in the range of'400 to 800 feet per second. At

higher velocities, the blade angle is preferably decreased to about 15 degrees at 1,000 feet per second blade speed.

From such an analysis, it becomes apparent that in most designs a stator could be added downstreanjifof the rotor to improve performance. Representative values for the blade chord-to-spacing ratio and the blade angles for a singlestorage pump, having one row of rotor blades and one row of stator blades, would be as follows:

Value Rotor Stator ble 1. 0 1. 0

a 20 degrees 10 degrees where V is the tangential blade velocity, R is the gas constant, and T is the absolute temperature in degrees Kelvin. Stresses in the aluminum rotor limit the maximum speed V at 1,000 feet per second. Values of \/2RT of 1,350 feet per second for air at room temperature and 1,150 feet per second for argon at room temperature were used. A value of Z up to approximately .75 can be attained with air, while for argon a value of .875 is reasonably possible under the assumed conditions. Values of Z increase either with a decrease in temperature or with an increase in molecular weight of the gas involved.

From FIG. 7, it will be seen that the pressure ratios in the free-molecular flow range are greatly increased over pressure ratios similarly obtained with blowers or turbine pumps at atmospheric pressures and increase rapidly for the higher values of the blade-speed ratios. This, of course, indicates that the rotor speeds should be as high as possible in order to get maximum tangential velocity of the blades. On the other hand, this velocity is limited by mechanical considerations involved in rotor design to resist centrifugal forces. Of course, constructing the rotor of higher strength-to-weight materials will allow blade speeds higher than 1,000 feet per second, if desired.

FIG. 7 indicates that for a throughflow ratio, W, of .1, a pressure ratio of between 6 and 8 is possible for a singlestage radial-flow molecular pump when pumping air or argon at room temperatures, within the design parameters assumed. This means that the radial-flow molecular pump by discharging into an oil diffusion pump can increase the mass flow rate capacity of the diffusion pump by a factor of between 6 and 8. This factor is also significant because the performance of an oil diffusion pump begins to fall off at inlet pressures less than the general order of 10- millimeters of mercury. This means that the molecular pump of the present design can operate in a pressure range unsuited to the diffusion pump but can reach an outlet pressure that is within the range of efficient operation of the diffusion pump.

The pump illustrated herein has been designed particularly to operate in a chamber wherein the pressure is to be maintained, in the presence of relatively large massfiow rates, at millimeters of mercury. By using a twostage pump as illustrated, the overall pressure ratio of the pump can be raised to 10 or more. This characteristic of the molecular pump makes it particularly suited to operate at an inlet pressure of the order of magnitude maintained in chamber 11 and deliver its output to an oil diffusion pump, since the output is then in the range of efficient operation of the diffusion pump.

For the pump design, the test chamber is approximately 40 inches in diameter and 36 inches in length. With a mass-flow rate of 2 grams of argon per hour, a vacuum pump system of high volumetric capacity is required, since at the pressure maintained, this represents approximately 56,200 cubic feet per minute of gas which must be evacuated from the test chamber in order to maintain the desired pressure. By establishing a design objective of a pressure ratio of 10:1, the molecular pump would then reduce the pumping requirements to the diffusion pump to 5,620 cubic feet per minute at 10 millimeters of mercury. It will be apparent from the above discussion that the volumetric capacity of the molecular pump can be increased as desired, since the pumping capacity is in general a function of the total pumping area on the rotor. For a given diameter of rotor and blade arrangement, the total pumping area can be increased by increasing the axial length of the rotor, gaining a corresponding increase in total pumping area. This illustrates one of the particular advantages of the molecular pump of this design in that it is free of some of the capacity limitations imposed by geometry upon pumps of other designs, particularly axial flow pumps.

Under these conditions, the following blade parameters were selected to obtain the desired pressure ratio, although it will be realized that these values can be varied as may be desired to effect a higher pressure ratio or a greater throughput of gas:

An absolute value of the blade spacing, s, is established taking into consideration the molecular mean free path I and the significant dimension L of the flow field in the chamber. For free flow molecular conditions to exist, the ratio, l/L, commonly referred to as Knudsen number, should be greater than 1. There is experimental evidence that the Knudsen number should be greater than 5 on the downstream side for maximum performance, while above 5 the performance of the molecular pump has been found to remain substantially constant. For the design of the rotor, argon gas at 10* millimeters of mercury and 25 C., the molecular mean free path I was calculated to be approximtaely 6 meters. Assuming a maximum pressure ratio of 20:1 across the two stages of the pump and a Knudsen number of 5, the blade spacing, s, which corresponds to the significant dimension L in the formula and in our flow fields, should be approximately 6 centimeters or 2.4 inches.

The pump described above is adapted to radially out ward flow; but it will be appreciated that the pump can be modified by reversing the direction of rotor revolution to adapt it to radially inward flow, in which case the pump evacuates the space surrounding the pump but contained within wall means 10. An arrangement of this character is illustrated diagrammatically in FIG. 2 in which pump 20a is adapted to pump gas from inside the chamber past the blades into the interior of the rotor. From the rotor interior, which then becomes the high pressure zone, gas flows through the coaxially located duct 55 to the inlet of pump 15 which exhausts the gas to a still higher pressure. Under these circumstances, the space inside wall means 10 becomes the source of gas to be pumped, the gas being generated by some means, not shown in the drawings, contained Within the chamber defined by the wall means.

FIGS. 812 inclusive are diagrams showing various possible configurations of the stator and rotor blade assemblies, demonstrating that the principle involved in the molecular pump can be embodied in different types of apparatus. FIGS. 8 and 9 illustrate arrangements in which the rotor and the stator are both supported at two positions, more particularly at the two opposite ends of the pump. In FIG. 8, the innermost row of blades is a rotating row, adapting the pump to radially outward flow, as previously described. FIG. 9 illustrates a construction in which the innermost row is a stationary row, while the outermost row is rotating, thus adapting the pump to radially inward flow. In each of these arrangements, the innermost and outermost rows are continuous from endto-end where the rows are supported. The first row adjacent the innermost row is supported centrally from a row farther out by a median support ring which extends through the intervening row or rows because the intervening row or rows are divided centrally to allow for passage of this support member. Such divided intervening row or rows are then supported at one end only, the supported ends being the opposite ends of the divided rows. This general characterization of the configuration of the blade assemblies is true regardless of the direction of gas flow through the pump. While normally the imperforate end walls 38 support the rotating blade assemblies, these wall members can become the supports for the stationary blades, especially with the arrangement of FIG. 9.

FIGS. 10 and 11 illustrate possible constructions in which both of the rotor and stator blade assemblies are supported at one end only. In the arrangement of FIG. 10, suited for radially outward flow, the rotor can be supported conveniently at the same end of the pump as the stator. This has an obvious structural advantage as it facilitates maintenance of concentricity of the blade rows. The relative positions of the rotor and stator blades can be changed to adapt the pump to radially inward flow, as in FIG. 11. It is possible also to utilize an arrangement in which the rotor is supported at one end of the pump and the stator at the other end, as shown in FIG. 11. However, support for both rotor and stator at the same end of the pump can be Worked out for either direction of gas flow.

FIG. 12 shows another possible variation which combines features shown in both FIG. 8 and 11. FIG. 12 may be considered in one sense as being a double rotor formed by placing two rotors as in FIG. 11 back-to-back. The two designs are similar to the extent that they each have one set of blades, in this case the stationary or stator blades, supported at only one end of the rows. However, the arrangement of FIG. 12 also resembles that shown in FIG. 9 in that one set of blades is centrally supported by a median support member passing between divided rows of blades. Gas inflow or outflow at the center of the pump can be either divided or unidirectional.

In all of the configurations described above, the blades are preferably flat, or substantially so, as described; but it is within the scope of the invention to curve the blades slightly, either in a transverse or in a longitudinal plane. In the latter case, the rotor becomes barrel-shaped instead of cylindrical as illustrated, and the degree of curvature may be such as to greatly reduce or substantially eliminate bending stresses and subject the blades only to tensile stresses.

From the foregoing discussion, it will be apparent that various changes in the exact size, shape and dimension of the parts of the pump may be made without departing from the spirit and scope of the present invention. Accordingly, it is to be understood that the foregoing description is considered as being illustrative of, rather than limitative upon, the invention as defined by the appended claims.

We claim:

1. The method of pumping gas from a first zone in which free molecular flow conditions exist to a second zone of higher pressure, that includes the steps of:

producing a source of rarefied gas;

supplying gas from said source to a first zone to be maintained under conditions of free molecular flow at pressures less than 1O mm. of mercury;

and moving a series of spaced, substantially flat blades in a direction at an agle of about 20 to the principal surfaces of the blade and in contact with gas from said first zone whereby the blades encounter the molecules with a higher probability of the molecules rebounding from the blades from the first zone into the second zone than back into the first zone.

about .75

when the gas is air at room temperature,

V is the tangential velocity,

R is the gas constant, and

T is .the absolute temperature in degrees Kelvin.

3. The combination comprising:

an enclosed source of rarefied gas to be pumped supplying gas under conditions of free molecular flow at pressures less than 10* mm. of mercury;

and a radial flow molecular pump having a central inlet communicating with said source and including a rotor having a pair of axially spaced end walls and a plurality concentric rings of blades extending axially between said walls,

said blades being thin, substantially flat members disposed at an angle of about 20 to a tangent at the center of each blade;

and a plurality of concentric rings of thin, substantially fiat stator blades arranged with each ring of stator blades concentric with and downstream from one of said rings of rotor blades, the blades in each ring of stator blades being disposed at an angle of about 20 to a target at the center of each blade but oppositely inclined relative to the rotor blades.

4. The combination as in claim 3 in which the innermost row of blades extends axially the full distance between spaced supports therefor;

the first adjacent row of blades extends axially less than said distance;

the second adjacent row of blades is divided centrally into two portions each supported at one end only;

and the outermost row of blades extends axially the full distance between spaced supports therefor and is connected centrally to said first adjacent row of blades to support said first adjacent row.

5. The combination as in claim 3 in which the innermost row of blades is a row of rotor blades supported at its ends on said spaced end walls;

the first downstream row of blades is a row of stator blades shorter than said innermost row;

the second downstream row of blades is divided centrally into two portions each supported at one end on one of said end walls;

the outermost row of blades is a row of stator blades supported at its ends;

and which includes means supporting the first-mentioned row of stator blades from the outermost row of stator blades, said supporting means passing through said divided row of rotor blades.

6. The combination as in claim 3 in which the rows of stator blades are supported at one end only;

and the rows of rotor blades are supported at one end only,

the rows of stator and rotor blades extending oppositely from their respective supports.

7. The combination as in claim 3 in which the rows of stator blades are supported at one end from one of said end walls;

and the rows of rotor blades are supported at one end from the other of said end walls,

the rows of stator blades and the rows of rotor blades extending from their respective end walls toward and in close proximity to the other of said end walls.

(References on following page) 11 12 References Cited FOREIGN PATENTS UNITED STATES PATENTS 21,904 1906 Great Britain.

29,262 7/1860 Fitzpatrick 230-134 21905 1906 Great m 889,164 5/1908 We1ch 103 110 5 28,401 1397 Great mm- 1,069,408 8/1913 Gaede 103-84 608,705 9/1948 Great mi 1,519,245 12/1924 Fechheimer 230 134 8761612 Great Bntam- DONLEY J. STOCKING, Primary Examiner. 2,335,445 11/1943 Richard 103 110 HENRY F. RADUAZO, MARTIN P. SCHWADRON, 2,826,353 3/1958 Auwarter et a1. 230-101 10 Examiners.

2,954,157 9/1960 Eckberg 230124 

1. THE METHOD OF PUMPING GAS FROM A FIRST ZONE IS WHICH FREE MOLECULAR FLOW CONDITIONS EXIST TO A SECOND ZONE OF HIGHER PRESSURE, THAT INCLUDES THE STEPS OF: PRODUCING A SOURCE OF RAREFIED GAS; SUPPLYING GAS FROM SAID SOURCE TO A FIRST ZONE TO BE MAINTAINED UNDER CONDITIONS OF FREE MOLECULAR FLOW AT PRESSURES LESS THAN 10-4 MM. OF MERCURY; AND MOVING A SERIES OF SPACED, SUBSTANTIALLY FLAT BLADES IN A DIRECTION AT AGLE OF ABOUT 200 TO THE PRINCIPAL SURFACES OF THE BLADE AND IN CONTACT WITH GAS FROM SAID FIRST ZONE WHEREBY THE BLADES ENCOUNTER 